1. Field of the Invention
This invention relates generally to the treatment of coal tailings, and, more particularly, provides a method for improved belt press dewatering for coal.
2. Brief Description of the Prior Art
In the mining of coal, various types of shale and clay are produced along with the coal. To increase the heating value of the coal and to reduce hauling costs, a coal washing process is normally used. In this process, the coal is graded and the coarse sizes, generally greater than a 1/4 of an inch in diameter, are then fed through a slurry vat in which the density of the vat media is closely controlled. The coal floats in the heavy media of the vat while heavier rocks sink to the bottom.
The smaller-sized fractions, less than a 1/4 inch in diameter, may be processed in spiral concentrators, shaking tables, hydrocyclones. The smallest particles, less than 1/2 millimeter in diameter, are processed by froth flotation. In each of these steps, more coal is recovered and the refuse is dewatered as well as possible. While spiral concentrations, shaking tables, and hydrocyclones utilize differences in specific gravity, separation based on differences in specific gravity is inefficient for smaller-sized fractions. Thus, in these lower size ranges, differences in the surfaces characteristics between high ash particles and the cleaner low ash particles are relied on in order to separate the particles. This process is called froth flotation.
In the froth flotation process, the fine coal particles are fed to an agitation tank in the form of a slurry having solids levels typically at about 3 to 8 percent but sometimes as high as about 15 to 20 percent. The slurry is conditioned with a class of chemicals called collectors which selectively coat and thereby impart hydrophobicity to the coal particles while leaving the higher ash fractions untouched. A frother is then added and the slurry is conditioned. Air is then bubbled through the mixture. The hydrophobized coal particles stick to the bubbles and rise to the top of the tank in the form of a froth. High ash hydrophilic fractions remain in the slurry and are called tailings. The tailings, which typically constitute about 2 to 4 percent solids are then sent to a thickener to be settled, so that the water in the slurry may be reused in the process to thereby minimize or eliminate effluent flow. It is thus highly desirable to dewater this refuse slurry as efficiently as possible.
In the thickener, coagulants and flocculants are used to help dewater the slurry. Coagulants are added in order to neutralize charges on the particles in the slurry. The charge neutralized particles form pin or micro flocs. One commonly used and generally effective coagulant is polydiallyldimethylammonium chloride ("DADMAC") having a molecular weight of about 100,000 da. Treatment with such coagulants is then followed with treatment with a flocculant.
Flocculants work by gathering together the floc particles in a net, bridging from one surface to the other and binding the individual particles into large agglomerants. Flocculation not only increases the size of the flocced particles, it also affects the physical nature of the floc, so that the slurry will dewater at a faster rate because of the reduction of the gelatinous structure of the floc.
In the flocculation of tailing slurries, typical flocculants include anionic high molecule weight polymers, copolymers of acrylamide and acrylic acid (acrylates), polyacrylamides and polyacrylates having molecular weights from about 1,000,000 to about 20,000,000 da.
The flocs settle to the bottom of the thickener, and, after a short period of time form a compact bed. This bed is discharged as tailings from the bottom of the thickener in the form of a 30 to 40 percent solids slurry. The tailings are either sent to belt presses where they are further dewatered and then disposed of in tailing ponds or sent directly to tailings ponds.
Further dewatering of the coal slurry is usually accomplished using a twin belt press. The press uses a combination of gravity and gradually applied pressure to mechanically dewater the slurry. Coagulants and flocculants are added to the slurry to facilitate the dewatering process. The coagulant and flocculant improve dewatering by improving the drainage of water from the slurry. Although the order of addition can be varied, generally, for twin belt press dewatering operations, the flocculant is often added to the slurry before the coagulant is added.